Light olefins, i.e. ethylene and propylene, are two important kinds of basic chemical raw materials, and the demand thereof is increasing. Generally, ethylene and propylene are produced via a petroleum scheme. However, the costs for producing ethylene and propylene from petroleum resources are increasing due to limited supply and relatively high price of petroleum resources. In recent years, techniques for preparing ethylene and propylene by converting substituent raw materials have been greatly developed. More and more attentions have been paid to the process of methanol-to-olefins (MTO), and the production scale of megatonnage has been achieved. As the world economy develops, the demand for light olefins, particularly propylene, is increasing day by day. It is reported as the analysis of CMAI Corporation that the demand for ethylene will increase at an average rate of 4.3% per year and the demand for propylene will increase at an average rate of 4.4% per year until 2016. Due to high-speed increase of the economy in China, all of the annual increase rates of the demand for ethylene and propylene in China exceed the average level of the world.
In early 1980s, UCC Corporation successfully developed SAPO series molecular sieves. Among others, SAPO-34 molecular sieve catalyst exhibits excellent catalytic performance when it is used in MTO reaction, and has very high selectivity for light olefins and very high activity. However, after the catalyst has been used for a period of time, the activity is lost due to carbon deposition. A remarkable induction period is present in the use of the SAPO-34 molecular sieve catalyst. In the induction period, the selectivity for olefins is relatively low and the selectivity for alkanes is relatively high. As the reaction time increases, the selectivity for light olefins gradually increases. After the induction period, the catalyst maintains high selectivity and high activity in a certain period of time. With further prolong of the time, however, the activity of the catalyst rapidly decreases.
U.S. Pat. No. 6,166,282 discloses a technique and a reactor for converting methanol to light olefins, which use a fast fluidized bed reactor, wherein after the completion of a reaction in a dense phase reaction zone having a relatively low gas speed, the gas phase rises to a fast separation zone having an inner diameter which rapidly becomes smaller, and most of the entrained catalyst is preliminarily separated using a special gas-solid separation apparatus. Since the product gas and the catalyst are rapidly separated after reaction, a secondary reaction is effectively prevented. Upon analog computation, the inner diameter of the fast fluidized bed reactor and the catalyst inventory required are both greatly reduced, compared to the conventional bubbling fluidized bed reactors. However, the carbon based yields of light olefins in this method are all typically about 77%, and there are problems concerning relatively low yields of light olefins.
CN101402538B discloses a method for increasing the yield of light olefins. This method provides a second reaction zone on the upper part of a first reaction zone for converting methanol to light olefins, and the diameter of the second reaction zone is greater than that of the first reaction zone to increase the residence time of the product gas from the outlet of the first reaction zone in the second reaction zone, such that the unreacted methanol, the generated dimethyl ether, and hydrocarbons having 4 or more carbons continue to react so as to achieve the object of increasing the yield of light olefins. This method may increase the yield of light olefins to some extent. However, since the catalyst come out from the first reaction zone has already carried a relatively great amount of deposited carbon and relatively high catalyst activity is required to crack hydrocarbons having 4 or more carbons, the conversion efficiencies of hydrocarbons having 4 or more carbons in the second reaction zone in this method are still relatively low, leading to a lower yield of light olefins.
CN102276406A discloses a method for increasing the production of propylene. This technique provides three reaction zones, wherein a first fast bed reaction zone is used for converting methanol to olefins, and a lift pipe reaction zone and a second fast bed reaction zone are connected in series to convert ethylene, hydrocarbons having 4 or more carbons, and unreacted methanol or dimethyl ether. In this patent application, the residence times of substances, such as hydrocarbons having 4 or more carbons, etc., in the lift pipe reaction zone and in the second fast bed reaction zone are relatively short and the conversion efficiencies are relatively low, such that the yield of propylene is relatively low.
CN102875289A discloses a fluidized bed reaction device with a lift pipe reactor arranged therein, which is used for increasing the yield of light olefins. A first raw material is passed into a fluidized bed reaction zone and is brought into contact with a catalyst to generate a product comprising light olefins, and at the meanwhile a spent catalyst is formed; a part of the spent catalyst is passed into a regenerator for regeneration to form a regenerated catalyst, and the other part of the spent catalyst is passed into a lift pipe with an outlet end located inside the reaction zone and is brought into contact with a second raw material so as to lift the spent catalyst into the reaction zone; and the regenerated catalyst is returned to the reaction zone of the fluidized bed reactor. Since the reaction device disclosed in this patent application does not comprise a stripping portion, the spent catalyst will be passed into the regenerator with carrying a part of the product gas, which is combusted with oxygen to reduce the yield of light olefins.
The technique for preparing olefins from methanol disclosed in CN102875296A provides three reaction zones, which are a fast bed, a downer, and a lift pipe. Since the catalyst is circulated among the regenerator, the fast bed, the lift pipe, and the downer, the flow direction is extremely complicated, the distribution and the control of the flow rate are extremely difficult, and the activity of catalyst greatly varies.
As well known in the art, the selectivity for light olefins is closely associated with the amount of carbon deposition on the catalyst. A certain amount of carbon deposition on SAPO-34 catalyst is needed to ensure a high selectivity for light olefins. Main reactors used in current MTO process are fluidized beds. The fluidized bed is close to a perfect mixing flow reactor, which has a wide distribution of carbon deposition on catalyst and is not advantageous for increasing the selectivity for light olefins. Since the catalyst-to-alcohol ratio is very small and the coke yield is relatively low in the MTO process, in order to achieve a lager and controllable catalyst circulation volume, it is required to control the amount of carbon deposition and the uniformity of carbon content on the catalyst to a certain level in the regeneration zone, thereby achieving the object of controlling the amount of carbon deposition and the uniformity of carbon content on the catalyst in the reaction zone. Therefore, it is a key technique in the MTO process to control the amount of carbon deposition and the uniformity of carbon content of the catalyst in the reaction zone to a certain level.
Furthermore, in the process of preparation of a light olefin using methanol, byproduct such as a hydrocarbon with four or more carbons etc. will be produced inevitably, and the carbon-based selectivity for the hydrocarbon with four or more carbons is generally around 10 wt %. If these byproducts can be further converted into target product, the yield of light olefin and the economical efficiency of the process will be improved. Our research has demonstrated that carbon-free SAPO-34 molecular sieve catalyst has excellent effect when using in the preparation of a light olefin by conversion of a hydrocarbon with four or more carbons, and may achieve the pre-carbon deposition on the SAPO-34 molecular sieve catalyst, thereby improving its selectivity for light olefin. A light olefin is very sensitive to the process parameters such as reaction temperature etc. For example, the temperature for regenerating the catalyst is generally higher than 550° C., which is much higher than the temperature of reaction zone, and the local over-temperature at the inlet for regenerated catalyst would decrease the selectivity for light olefin.
The factors such as distribution non-uniformity of carbon deposition on catalyst, carbon content being difficult to control and large fluctuation of reaction temperature are all disadvantageous for the improvement of the light olefin yield. In order to solve the problems described above, some researchers propose the techniques, such as providing an upper and a lower reaction zones in a fluidized bed, two fluidized beds connected in series, and a fluidized bed, a lift pipe, and a downer connected in series, and certain advantageous effects have been obtained. However, the complexity and the difficulty for controlling the MTO process are increased at the meanwhile. The present disclosure provides a solution in which multiple secondary reaction zones (regeneration zones) are formed by providing inner members in the dense phase fluidized bed for improving the yield of light olefin.